Wyeth has notified health care professionals of changes to the Rapamune (sirolimus) prescribing information related to changes in the performance of an immunoassay used for therapeutic drug monitoring (TDM) of the medication, according to the FDA.

The TDM results reported from the assay are both assay- and laboratory dependent. Additionally, the results may change over time. “Therefore, adjustment to the targeted therapeutic range must be made with a detailed knowledge of the site-specific assay used,” the FDA noted.

Sirolimus whole blood concentrations can be measured with either chromatographic or immunoassay methodologies, the agency explained, adding that these methodologies are not directly interchangeable. Moreover, the measured sirolimus whole blood concentrations depend on the type of assay used.

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“As such, if different assays are used in monitoring a single patient, the dose of Rapamune might be adjusted improperly with potential consequences, such as allograft rejection if drug exposure is too low or toxic side effects if exposure is too high.”

It is critical that clinicians caring for a patient on sirolimus maintain communication with their laboratory to determine whether the assay used for measuring sirolimus concentrations has been changed.